Bandpass filter with frequency separation between shunt and series resonators set by dielectric layer thickness

ABSTRACT

An acoustic filter includes a piezoelectric plate on a substrate. Portions of the piezoelectric plate form one or more diaphragms, each diaphragm spanning a respective cavity in the substrate. A conductor pattern on a front surface of the piezoelectric plate includes interdigital transducers (IDTs) of acoustic resonators including a shunt resonator and a series resonator. Interleaved fingers of each IDT are on a diaphragm of the one or more diaphragms. A first dielectric layer with a first thickness is between the fingers of the IDT of the shunt resonator, and a second dielectric layer with a second thickness less than the first thickness is between the fingers of the IDT of the series resonator. The piezoelectric plate and the IDTs are configured such that radio frequency signals applied to the IDTs excite respective primary shear acoustic modes within the diaphragms.

RELATED APPLICATION INFORMATION

This patent is a continuation of application Ser. No. 17/109,812,entitled TRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATORS AND FILTERS,filed Dec. 2, 2020, which is a continuation-in-part of application Ser.No. 16/689,707, entitled BANDPASS FILTER WITH FREQUENCY SEPARATIONBETWEEN SHUNT AND SERIES RESONATORS SET BY DIELECTRIC LAYER THICKNESS,filed Nov. 20, 2019, now U.S. Pat. No. 10,917,070, which is acontinuation of application Ser. No. 16/230,443, entitledTRANSVERSELY-EXCITED FILM BULK ACOUSTIC RESONATOR, filed Dec. 21, 2018,now U.S. Pat. No. 10,491,192, which claims priority from the followingprovisional patent applications: application 62/685,825, filed Jun. 15,2018, entitled SHEAR-MODE FBAR (XBAR); application 62/701,363, filedJul. 20, 2018, entitled SHEAR-MODE FBAR (XBAR); application 62/741,702,filed Oct. 5, 2018, entitled 5 GHZ LATERALLY-EXCITED BULK WAVE RESONATOR(XBAR); application 62/748,883, filed Oct. 22, 2018, entitled SHEAR-MODEFILM BULK ACOUSTIC RESONATOR; and application 62/753,815, filed Oct. 31,2018, entitled LITHIUM TANTALATE SHEAR-MODE FILM BULK ACOUSTICRESONATOR. All these applications are incorporated herein by reference.

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by anyone of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

BACKGROUND Field

This disclosure relates to radio frequency filters using acoustic waveresonators, and specifically to filters for use in communicationsequipment.

Description of the Related Art

A radio frequency (RF) filter is a two-port device configured to passsome frequencies and to stop other frequencies, where “pass” meanstransmit with relatively low signal loss and “stop” means block orsubstantially attenuate. The range of frequencies passed by a filter isreferred to as the “pass-band” of the filter. The range of frequenciesstopped by such a filter is referred to as the “stop-band” of thefilter. A typical RF filter has at least one pass-band and at least onestop-band. Specific requirements on a passband or stop-band depend onthe specific application. For example, a “pass-band” may be defined as afrequency range where the insertion loss of a filter is better than adefined value such as 1 dB, 2 dB, or 3 dB. A “stop-band” may be definedas a frequency range where the rejection of a filter is greater than adefined value such as 20 dB, 30 dB, 40 dB, or greater depending onapplication.

RF filters are used in communications systems where information istransmitted over wireless links. For example, RF filters may be found inthe RF front-ends of cellular base stations, mobile telephone andcomputing devices, satellite transceivers and ground stations, IoT(Internet of Things) devices, laptop computers and tablets, fixed pointradio links, and other communications systems. RF filters are also usedin radar and electronic and information warfare systems.

RF filters typically require many design trade-offs to achieve, for eachspecific application, the best compromise between performance parameterssuch as insertion loss, rejection, isolation, power handling, linearity,size and cost. Specific design and manufacturing methods andenhancements can benefit simultaneously one or several of theserequirements.

Performance enhancements to the RF filters in a wireless system can havebroad impact to system performance. Improvements in RF filters can beleveraged to provide system performance improvements such as larger cellsize, longer battery life, higher data rates, greater network capacity,lower cost, enhanced security, higher reliability, etc. Theseimprovements can be realized at many levels of the wireless system bothseparately and in combination, for example at the RF module, RFtransceiver, mobile or fixed sub-system, or network levels.

High performance RF filters for present communication systems commonlyincorporate acoustic wave resonators including surface acoustic wave(SAW) resonators, bulk acoustic wave (BAW) resonators, film bulkacoustic wave resonators (FBAR), and other types of acoustic resonators.However, these existing technologies are not well-suited for use at thehigher frequencies and bandwidths proposed for future communicationsnetworks.

The desire for wider communication channel bandwidths will inevitablylead to the use of higher frequency communications bands. Radio accesstechnology for mobile telephone networks has been standardized by the3GPP (3^(rd) Generation Partnership Project). Radio access technologyfor 5^(th) generation mobile networks is defined in the 5G NR (newradio) standard. The 5G NR standard defines several new communicationsbands. Two of these new communications bands are n77, which uses thefrequency range from 3300 MHz to 4200 MHz, and n79, which uses thefrequency range from 4400 MHz to 5000 MHz. Both band n77 and band n79use time-division duplexing (TDD), such that a communications deviceoperating in band n77 and/or band n79 use the same frequencies for bothuplink and downlink transmissions. Bandpass filters for bands n77 andn79 must be capable of handling the transmit power of the communicationsdevice. WiFi bands at 5 GHz and 6 GHz also require high frequency andwide bandwidth. The 5G NR standard also defines millimeter wavecommunication bands with frequencies between 24.25 GHz and 40 GHz.

DESCRIPTION OF THE DRAWINGS

FIG. 1 includes a schematic plan view and two schematic cross-sectionalviews of a transversely-excited film bulk acoustic resonator (XBAR).

FIG. 2 is an expanded schematic cross-sectional view of a portion of theXBAR of FIG. 1 .

FIG. 3A is an alternative schematic cross-sectional view of the XBAR ofFIG. 1 .

FIG. 3B is another alternative schematic cross-sectional view of theXBAR of FIG. 1 .

FIG. 3C is an alternative schematic plan view of an XBAR.

FIG. 4 is a graphic illustrating a shear horizontal acoustic mode in anXBAR.

FIG. 5 is a chart of the admittance of a simulated XBAR.

FIG. 6 is a chart comparing the admittances of three simulated XBARswith different dielectric layers.

FIG. 7 is a chart comparing the admittances of four simulated XBARs withdifferent dielectric layer thicknesses.

FIG. 8 is a plot showing the effect of piezoelectric plate thickness onresonance frequency of an XBAR.

FIG. 9 is a plot showing the effect of front dielectric layer thicknesson resonance frequency of an XBAR.

FIG. 10 is a plot showing the effect of IDT finger pitch on resonancefrequency of an XBAR.

FIG. 11 is a chart comparing the admittances of XBARs on LiNbO3 andLiTaO3 plates.

FIG. 12 is a chart of the measured admittance of an XBAR.

FIG. 13 is another chart of the measured admittance of an XBAR.

FIG. 14 is a schematic circuit diagram and layout of a filter usingXBARs.

FIG. 15 is a graph of the transfer curve (S21) of an embodiment of thefilter of FIG. 12 .

FIG. 16 is a graph of the transfer curve (S21) of another embodiment ofthe filter of FIG. 12 .

Throughout this description, elements appearing in figures are assignedthree-digit or four-digit reference designators, where the two leastsignificant digits are specific to the element and the one or two mostsignificant digit is the figure number where the element is firstintroduced. An element that is not described in conjunction with afigure may be presumed to have the same characteristics and function asa previously-described element having the same reference designator.

DETAILED DESCRIPTION

Description of Apparatus

FIG. 1 shows a simplified schematic top view and orthogonalcross-sectional views of a transversely-excited film bulk acousticresonator (XBAR) 100. XBAR resonators such as the resonator 100 may beused in a variety of RF filters including band-reject filters, band-passfilters, duplexers, and multiplexers. XBARs are particularly suited foruse in filters for communications bands with frequencies above 3 GHz.

The XBAR 100 is made up of a thin film conductor pattern formed on asurface of a piezoelectric plate 110 having substantially parallel frontand back surfaces 112, 114, respectively. The piezoelectric plate is athin single-crystal layer of a piezoelectric material such as lithiumniobate, lithium tantalate, lanthanum gallium silicate, gallium nitride,or aluminum nitride. The piezoelectric plate is cut such that theorientation of the X, Y, and Z crystalline axes with respect to thefront and back surfaces is known and consistent. In the examplespresented in this patent, the piezoelectric plates are Z-cut, which isto say the Z axis is normal to the surfaces. However, XBARs may befabricated on piezoelectric plates with other crystallographicorientations including rotated Z-cut and rotated Y-cut.

A portion of the back surface 114 of the piezoelectric plate 110 isattached to a substrate 120 that provides mechanical support to thepiezoelectric plate 110. A cavity 140 is formed in the substrate.“Cavity” has its conventional meaning of “an empty space within a solidbody.” The cavity 140 may be a hole completely through the substrate 120(as shown in Section A-A and Section B-B) or a recess in the substrate120. The cavity 140 may be formed, for example, by selective etching ofthe substrate 120. The dashed line 145 in the plan view is the perimeterof the cavity 140, which is defined by the intersection of the cavityand the back surface 114 of the piezoelectric plate 110. As shown inFIG. 1 , the perimeter 145 of the cavity 140 has a rectangular shapewith an extent greater than the aperture AP and length L of the IDT 130.A cavity of an XBAR may have a different shape, such as a regular orirregular polygon. The cavity of an XBAR may more or fewer than foursides, which may be straight or curved.

The portion of the piezoelectric plate 110 outside of the perimeter ofthe cavity 145 is attached to the substrate. This portion may bereferred to as the “supported portion” of the piezoelectric plate. Theportion 115 of the piezoelectric plate 110 within the perimeter of thecavity 145 is suspended over the cavity 140 without contacting thesubstrate 120. The portion 115 of the piezoelectric plate 110 that spansthe cavity 140 will be referred to herein as the “diaphragm” 115 due toits similarity to the diaphragm of a microphone.

The substrate 120 may be, for example, silicon, sapphire, quartz, orsome other material. The supported portion of the piezoelectric plate110 may be bonded to the substrate 120 using a wafer bonding process, orgrown on the substrate 120, or attached to the substrate in some othermanner. The piezoelectric plate may be attached directly to thesubstrate or may be attached to the substrate via one or moreintermediate material layer.

The conductor pattern of the XBAR 100 includes an interdigitaltransducer (IDT) 130. The IDT 130 includes a first plurality of parallelfingers, such as finger 136, extending from a first busbar 132 and asecond plurality of fingers extending from a second busbar 134. The term“busbar” is commonly used to identify the electrodes that connect thefingers of an IDT. The first and second pluralities of parallel fingersare interleaved. The interleaved fingers overlap for a distance AP,commonly referred to as the “aperture” of the IDT. The center-to-centerdistance L between the outermost fingers of the IDT 130 is the “length”of the IDT.

The first and second busbars 132, 134 serve as the terminals of the XBAR100. A radio frequency or microwave signal applied between the twobusbars 132, 134 of the IDT 130 excites an acoustic wave within thepiezoelectric plate 110. As will be discussed in further detail, theexcited acoustic wave is a bulk shear wave that propagates in thedirection normal to the surface of the piezoelectric plate 110, which isalso normal, or transverse, to the direction of the electric fieldcreated by the IDT fingers. Thus, the XBAR is considered atransversely-excited film bulk wave resonator.

The rectangular area defined by the length L and the aperture AP isconsidered the “transducer area”. Substantially all the conversionbetween electrical and acoustic energy occurs within the transducerarea. The electric fields formed by the IDT may extend outside of thetransducer area. The acoustic waves excited by the IDT are substantiallyconfined within the transducer area. Small amounts of acoustic energymay propagate outside of the transducer area in both the length andaperture directions. In other embodiments of an XBAR, the transducerarea may be shaped as a parallelogram or some other shape rather thanrectangular. All the overlapping portions of the IDT fingers and theentire transducer area are positioned on the diaphragm 115, which is tosay within the perimeter of the cavity defined by the dashed line 145.

For ease of presentation in FIG. 1 , the geometric pitch and width ofthe IDT fingers is greatly exaggerated with respect to the length(dimension L) and aperture (dimension AP) of the XBAR. A typical XBARhas more than ten parallel fingers in the IDT 130. An XBAR may havehundreds, possibly thousands, of parallel fingers in the IDT 130.Similarly, the thickness of the fingers in the cross-sectional views isgreatly exaggerated.

FIG. 2 shows a detailed schematic cross-sectional view of the XBAR 100.The piezoelectric plate 110 is a single-crystal layer of piezoelectricalmaterial having a thickness ts. ts may be, for example, 100 nm to 1500nm. When used in filters for LTE™ bands from 3.4 GHZ to 6 GHz (e.g.bands 42, 43, 46), the thickness ts may be, for example, 200 nm to 1000nm.

A front-side dielectric layer 214 may optionally be formed on the frontside of the piezoelectric plate 110. The “front side” of the XBAR is, bydefinition, the surface facing away from the substrate. The front-sidedielectric layer 214 has a thickness tfd. The front-side dielectriclayer 214 is formed between the IDT fingers 238. Although not shown inFIG. 2 , the front side dielectric layer 214 may also be deposited overthe IDT fingers 238. A back-side dielectric layer 216 may optionally beformed on the back side of the piezoelectric plate 110. The back-sidedielectric layer 216 has a thickness tbd. The front-side and back-sidedielectric layers 214, 216 may be a non-piezoelectric dielectricmaterial, such as silicon dioxide or silicon nitride. tfd and tbd maybe, for example, 0 to 500 nm. tfd and tbd are typically less than thethickness ts of the piezoelectric plate. tfd and tbd are not necessarilyequal, and the front-side and back-side dielectric layers 214, 216 arenot necessarily the same material. Either or both of the front-side andback-side dielectric layers 214, 216 may be formed of multiple layers oftwo or more materials.

The IDT fingers 238 may be one or more layers of aluminum or asubstantially aluminum alloy, copper or a substantially copper alloy,beryllium, titanium, tungsten, chromium, molybdenum, gold, or some otherconductive material. Thin (relative to the total thickness of theconductors) layers of other metals, such as chromium or titanium, may beformed under and/or over the fingers to improve adhesion between thefingers and the piezoelectric plate 110 and/or to passivate orencapsulate the fingers. The busbars (132, 134 in FIG. 1 ) of the IDTmay be made of the same or different materials as the fingers.

Dimension p is the center-to-center spacing or “pitch” of the IDTfingers, which may be referred to as the pitch of the IDT and/or thepitch of the XBAR. Dimension w is the width or “mark” of the IDTfingers. The IDT of an XBAR differs substantially from the IDTs used insurface acoustic wave (SAW) resonators. In a SAW resonator, the pitch ofthe IDT is one-half of the acoustic wavelength at the resonancefrequency. Additionally, the mark-to-pitch ratio of a SAW resonator IDTis typically close to 0.5 (i.e. the mark or finger width is aboutone-fourth of the acoustic wavelength at resonance). In an XBAR, thepitch p of the IDT is typically 2.5 to 10 times the width w of thefingers. In addition, the pitch p of the IDT is typically 2.5 to 25times the thickness is of the piezoelectric slab 212. The width of theIDT fingers in an XBAR is not constrained to one-fourth of the acousticwavelength at resonance. For example, the width of XBAR IDT fingers maybe 500 nm or greater, such that the IDT can be fabricated using opticallithography. The thickness tm of the IDT fingers may be from 100 nm toabout equal to the width w. The thickness of the busbars (132, 134 inFIG. 1 ) of the IDT may be the same as, or greater than, the thicknesstm of the IDT fingers.

FIG. 3A and FIG. 3B show two alternative cross-sectional views along thesection plane A-A defined in FIG. 1 . In FIG. 3A, a piezoelectric plate310 is attached to a substrate 320. A cavity 340, which does not fullypenetrate the substrate 320, is formed in the substrate under theportion of the piezoelectric plate 310 containing the IDT of an XBAR.The cavity 340 may be formed, for example, by etching the substrate 320before attaching the piezoelectric plate 310. Alternatively, the cavity340 may be formed by etching the substrate 320 with a selective etchantthat reaches the substrate through one or more openings 350 provided inthe piezoelectric plate 310.

In FIG. 3B, the substrate 320 includes a base 322 and an intermediatelayer 324 disposed between the piezoelectric plate 310 and the base 322.For example, the base 322 may be silicon and the intermediate layer 324may be silicon dioxide or silicon nitride or some other material. Acavity 340 is formed in the intermediate layer 324 under the portion ofthe piezoelectric plate 310 containing the IDT of an XBAR. The cavity340 may be formed, for example, by etching the intermediate layer 324before attaching the piezoelectric plate 310. Alternatively, the cavity340 may be formed by etching the intermediate layer 324 with a selectiveetchant that reaches the substrate through one or more openings providedin the piezoelectric plate 310.

FIG. 3C is a schematic plan view of another XBAR 360. The XBAR 360includes an IDT formed on a piezoelectric plate 310. The piezoelectricplate 310 is disposed over a cavity 380 in a substrate. In this example,the cavity 380 has an irregular polygon shape such that none of theedges of the cavity are parallel, nor are they parallel to theconductors of the IDT. A cavity may have a different shape with straightor curved edges.

FIG. 4 is a graphical illustration of the primary acoustic mode ofinterest in an XBAR. FIG. 4 shows a small portion of an XBAR 400including a piezoelectric plate 410 and three interleaved IDT fingers430. An RF voltage is applied to the interleaved fingers 430. Thisvoltage creates a time-varying electric field between the fingers. Thedirection of the electric field is lateral, or parallel to the surfaceof the piezoelectric plate 410, as indicated by the arrows labeled“electric field”. Due to the high dielectric constant of thepiezoelectric plate, the electric field is highly concentrated in theplate relative to the air. The lateral electric field introduces sheardeformation, and thus strongly excites shear-mode acoustic waves, in thepiezoelectric plate 410. In this context, “shear deformation” is definedas deformation in which parallel planes in a material remain paralleland maintain a constant distance while translating relative to eachother. “Shear acoustic waves” are defined as acoustic waves in a mediumthat result in shear deformation of the medium. The shear deformationsin the XBAR 400 are represented by the curves 460, with the adjacentsmall arrows providing a schematic indication of the direction andmagnitude of atomic motion. The degree of atomic motion, as well as thethickness of the piezoelectric plate 410, have been greatly exaggeratedfor ease of visualization. While the atomic motions are predominantlylateral (i.e. horizontal as shown in FIG. 4 ), the direction of acousticenergy flow of the excited shear acoustic waves is substantiallyvertical, normal to the surface of the piezoelectric plate, as indicatedby the arrow 465.

Considering FIG. 4 , there is essentially no electric field immediatelyunder the IDT fingers 430, and thus acoustic modes are only minimallyexcited in the regions 470 under the fingers. There may be evanescentacoustic motions in these regions. Since acoustic vibrations are notexcited under the IDT fingers 430, the acoustic energy coupled to theIDT fingers 430 is low (for example compared to the fingers of an IDT ina SAW resonator), which minimizes viscous losses in the IDT fingers.

An acoustic resonator based on shear acoustic wave resonances canachieve better performance than current state-of-the artfilm-bulk-acoustic-resonators (FBAR) and solidly-mounted-resonatorbulk-acoustic-wave (SMR BAW) devices where the electric field is appliedin the thickness direction. In such devices, the acoustic mode iscompressive with atomic motions and the direction of acoustic energyflow in the thickness direction. In addition, the piezoelectric couplingfor shear wave XBAR resonances can be high (>20%) compared to otheracoustic resonators. Thus, high piezoelectric coupling enables thedesign and implementation of microwave and millimeter-wave filters withappreciable bandwidth.

FIG. 5 is a chart 500 with a plot 510 of the normalized magnitude of theadmittance (on a logarithmic scale) as a function of frequency of anXBAR simulated using finite element method (FEM) simulation techniques.In the simulated XBAR, the piezoelectric plate is Z-cut (i.e. Z axisnormal to the plate) lithium niobate. The IDT fingers are aluminum. TheIDT is oriented such that the y-axis of the piezoelectric plate isnormal to the IDT fingers. The substrate supporting the piezoelectricplate is silicon with a cavity formed completely through the silicon (asshown in FIG. 1 ). Losses in the piezoelectric plate and IDT fingerswere simulated using standard material parameters. The simulatedphysical dimensions are as follows: is =400 nm; tfd=0; tbd=0; tm=100 nm;p=5 μm; w=500 nm. The admittance is normalized for a single pair of IDTfingers and an aperture of 1 meter. The admittance of an XBAR with N IDTfingers and an aperture A (in m) can be estimated by multiplying thenormalized admittance provided in FIG. 5 by (N−1)·A.

The simulated XBAR exhibits a resonance at a frequency FR 520 of 4693MHz and an anti-resonance at a frequency FAR 530 of 5306 MHz. The Q atresonance QR is 2645 and the Q at anti-resonance QAR is 4455. Theabsolute difference between FAR and FR is about 600 MHz, and thefractional difference is about 0.12. The acoustic coupling can beroughly estimated to 24%. Secondary resonances are evident in theadmittance curve at frequencies below FR and above FAR.

Acoustic RF filters usually incorporate multiple acoustic resonators.Typically, these resonators have at least two different resonancefrequencies. For example, an RF filter using the well-known “ladder”filter architecture includes shunt resonators and series resonators. Ashunt resonator typically has a resonance frequency below the passbandof the filter and an anti-resonance frequency within the passband. Aseries resonator typically has a resonance frequency within the passband and an anti-resonance frequency above the passband. In manyfilters, each resonator has a unique resonance frequency. An ability toobtain different resonance frequencies for XBARs made on the samepiezoelectric plate greatly simplifies the design and fabrication of RFfilters using XBARs.

FIG. 6 is a graph 600 comparing the normalized admittances, as functionsof frequency, of three XBARs with different dielectric layers. Theadmittance data, which is presented on a log scale, results fromtwo-dimensional simulation of a XBAR structure using the same materialsand dimensions (except for the dielectric layers) as the previousexample. The admittance is normalized for a single pair of IDT fingersand an aperture of 1 m. The solid line 610 is a plot of the normalizedadmittance per unit aperture for an XBAR with tfd=tbd=0 (i.e. an XBARwithout dielectric layers). The normalized admittance of this XBAR iscomparable to the normalized admittance plot in FIG. 5 , with slightdifferences due to the different simulation methodologies. The dashedline 620 is a plot of the normalized admittance for an XBAR with 100 nmof SiO2 on the front surface of the piezoelectric slab between the IDTfingers (tfd=100 nm and tbd=0). The addition of the SiO2 layer on thefront surface of the piezoelectric plate shifts the resonance frequencydown by about 500 MHz, or about 11%, compared to the XBAR with nodielectric layers. The dash-dot line 630 is a plot of the normalizedadmittance for an XBAR with 100 nm of SiO2 over the front surface of thepiezoelectric slab between the IDT fingers and 100 nm of SiO2 on theback surface of the piezoelectric slab (tfd=tbd=100 nm). The addition ofthe SiO2 layers on both surfaces of the piezoelectric plate shifts theresonance frequency down by about 900 MHz, or 20%, compared to the XBARwith no dielectric layers.

FIG. 7 is a graph 700 comparing the admittances, as functions offrequency, of four XBARs with different front-side dielectric layerthicknesses. The admittance data results from three-dimensionalsimulation of XBARs with the following parameter: is =400 nm; tfd=0, 30,60, 90 nm; tbd=0; tm=100 nm; p=4.2 μm; w=500 nm; AP=20 μm; and N (totalnumber of IDT fingers)=51. The substrate is Z-cut lithium niobate, theIDT conductors are aluminum, and the dielectric layers are SiO2.

The solid line 710 is a plot of the admittance of an XBAR with tfd=0(i.e. an XBAR without dielectric layers). The dashed line 720 is a plotof the admittance of an XBAR with tfd=30 nm. The addition of the 30 nmdielectric layer reduces the resonant frequency by about 145 MHzcompared to the XBAR without dielectric layers. The dash-dot line 730 isa plot of the admittance of an XBAR with tfd=60 nm. The addition of the60 nm dielectric layer reduces the resonant frequency by about 305 MHzcompared to the XBAR without dielectric layers. The dash-dot-dot line740 is a plot of the admittance of an XBAR with tfd=90 nm. The additionof the 90 nm dielectric layer reduces the resonant frequency by about475 MHz compared to the XBAR without dielectric layers. The frequencyand magnitude of the secondary resonances are affected differently thanthe primary shear-mode resonance.

Importantly, the presence of the dielectric layers of variousthicknesses has little or no effect on the piezoelectric coupling, asevidenced by the nearly constant frequency offset between the resonanceand anti-resonance of each XBAR.

FIG. 8 , FIG. 9 , and FIG. 10 are graphs showing the dependence,determined by simulation, of resonant frequency on XBAR physicalcharacteristics. Specifically, FIG. 8 is a graph 800 with curve 810showing resonant frequency as a function of piezoelectric platethickness ts with IDT finger pitch p=3 microns and no front-side orback-side dielectric layer (tfd=tbd=0). FIG. 9 is a graph 900 with curve910 showing resonant frequency as a function of front-side dielectriclayer thickness tfd for piezoelectric plate thickness ts=400 nm and IDTfinger pitch p=3 microns. FIG. 10 is a graph 1000 with curve 1010showing resonant frequency as a function of IDT finger pitch p withpiezoelectric plate thickness ts=400 nm and tfd=tbd=0. In all cases, thepiezoelectric substrate is Z-cut lithium niobate and the IDT fingerswere aluminum with a width w=500 nm and thickness tm=100 nm. Thefront-side dielectric layer, when present, is SiO2. The range of pitch pfrom 1.0 to 10.0 microns is equivalent to 2.5 to 25 times thepiezoelectric plate thickness and 2 to 20 time the IDT finger width.

FIG. 11 is a graph 1100 comparing the admittances, as functions offrequency, of two XBARs with different piezoelectric plate materials.The admittance data results from three-dimensional simulation of XBARswith the following parameter: is =415 nm; tfd=120 nm; tbd=0; tm=460 nm;p=4.5 μm; w=700 nm; AP=71 μm; and N (total number of IDT fingers)=221.The substrate is Z-cut lithium niobite or Z-cut lithium tantalate, theIDT electrodes are copper, and the dielectric layer is SiO2.

The solid line 1110 is a plot of the admittance of an XBAR on a lithiumniobate plate. The dashed line 1120 is a plot of the admittance of anXBAR on a lithium tantalate plate. Notably, the difference between theresonance and anti-resonance frequencies of the lithium tantalate XBARis about 5%, or half of the frequency difference of the lithium niobateXBAR. The lower frequency difference of the lithium tantalate XBAR isdue to the weaker piezoelectric coupling of the material. The measuredtemperature coefficient of the resonance frequency of a lithium niobateXBAR is about −71 parts-per-million per degree Celsius. The temperaturecoefficient of frequency (TCF) for lithium tantalate XBARs will be abouthalf that of lithium niobate XBARs. Lithium tantalate XBARs may be usedin applications that do not require the large filter bandwidth possiblewith lithium niobate XBARs and where the reduced TCF is advantageous.

FIG. 12 is a chart 1200 showing the measured admittance of anexperimental XBAR fabricated on a Z-cut lithium niobate plate with athickness of 400 nm. The IDT had a pitch of 5 μm, an aperture of 40 μm,and 101 IDT fingers. The IDT fingers were aluminum with a thickness of100 nm. The device did not include dielectric layers. The solid line1210 is the magnitude of admittance as a function of frequency. Theresonance frequency is 4617 MHz and the anti-resonance frequency is 5138MHz. The frequency difference is 521 MHz or more than 11% of theresonance frequency. The measured data has not been corrected for theeffects of the measurement system. Typically, correcting for themeasurement system increases the anti-resonance frequency and thedifferent between the anti-resonance and resonance frequencies.

FIG. 13 is a chart 1300 showing the measured admittance of anotherexperimental XBAR fabricated on a Z-cut lithium niobate plate with athickness of 400 nm. The IDT had a pitch of 5 μm, an aperture of 20 μm,and 51 fingers. The IDT fingers were aluminum with a thickness of 100nm. The device did not include dielectric layers. The solid line 1310 isthe magnitude of admittance as a function of frequency. The third andfifth harmonics of the primary XBAR resonance are visible at about 13.5GHz and 22.5 GHz, respectively. Resonances have been measured in otherXBARs at frequencies as high as 60 GHz.

FIG. 14 is a schematic circuit diagram for a high frequency band-passfilter 1400 using XBARs. The filter 1400 has a conventional ladderfilter architecture including three series resonators 1410A, 1410B,1410C and two shunt resonators 1420A, 1420B. The three series resonators1410A, 1410B, and 1410C are connected in series between a first port anda second port. In FIG. 14 , the first and second ports are labeled “In”and “Out”, respectively. However, the filter 1400 is bidirectional andeither port and serve as the input or output of the filter. The twoshunt resonators 1420A, 1420B are connected from nodes between theseries resonators to ground. All the shunt resonators and seriesresonators are XBARs.

The three series resonators 1410A, B, C and the two shunt resonators1420A, B of the filter 1400 are formed on a single plate 1430 ofpiezoelectric material bonded to a silicon substrate (not visible). Eachresonator includes a respective IDT (not shown), with at least thetransducer area of the IDT disposed over a cavity in the substrate. Inthis and similar contexts, the term “respective” means “relating thingseach to each”, which is to say with a one-to-one correspondence. In FIG.14 , the cavities are illustrated schematically as the dashed rectangles(such as the rectangle 1435). In this example, each IDT is disposed overa respective cavity. In other filters, the IDTs of two or moreresonators may be disposed over a single cavity.

In a ladder band-pass filter circuit, the anti-resonance frequencies ofthe series resonators 1410A, 1410B, 1410C are typically above the upperedge of the filter passband. Since each series resonator has very lowadmittance, approaching an open circuit, at its anti-resonancefrequency, the series resonators create transmission minimums (commoncalled “transmission zeros”) above the passband. The resonancefrequencies of the shunt resonators are typically below the lower bandedge of the filter pass band. Since each shunt resonator has very highadmittance, approaching a short circuit, at its resonance frequency, theshunt resonators create transmission minimums (common called“transmission zeros”) below the passband.

In some broadband filters, a dielectric layer may be formed on the topside, the bottom side, or both sides of the diaphragms of the shuntresonators to lower the resonance frequencies of the shunt resonatorsrelative to the anti-resonance frequencies of the series resonators.

FIG. 15 is a chart 1500 showing results from simulating a first bandpassfilter incorporating five XBARs. The schematic diagram of the firstfilter is the same as the filter 1400 of FIG. 14 . The XBARs are formedon a 0.4 micron thickness Z-cut lithium niobate plate. The substrate issilicon, the IDT conductors are aluminum, and there are no dielectriclayers. The other physical parameters of the resonators are provided inthe following table (all dimensions are in microns):

Series Resonators Shunt Resonators Parameter 1410A 1410B 1410C 1420A1420B p 1.475 1.475 1.525 3.52 3.52 w 0.53 0.53 0.515 0.51 0.51 AP 12.88.6 13.8 33 40 L 250 250 250 500 500

The performance of the first filter was simulated using a 3D finiteelement modeling tool. The curve 1510 is a plot of the magnitude of S21,the input-output transfer function, of the first filter as a function offrequency. The filter bandwidth is about 800 MHz, centered at 5.15 GHz.The simulated filter performance includes resistive and viscous losses.Tuning of the resonant frequencies of the various resonators isaccomplished by varying only the pitch and width of the IDT fingers.

FIG. 16 is a chart 1600 showing results from simulating a second filterusing five XBARs. The schematic diagram of the second filter is the sameas the filter 1400 of FIG. 14 . The XBARs are formed on a Z-cut lithiumniobate (0.4 μm thick) piezoelectric plate. The substrate is silicon,and the IDT electrodes are copper. Adjusting the resonant frequencies ofthe resonators is accomplished by varying the pitch and width of the IDTfingers and by providing a frequency-setting dielectric layer on thefront side between the IDT fingers of the shunt resonators to reducetheir frequencies relative to the frequencies of the series resonators.The other physical parameters of the resonators are provided in thefollowing table (all dimensions are in microns):

Series Resonators Shunt Resonators Parameter 1410A 1410B 1410C 1420A1420B p 4.189 4.07 4.189 4.2 4.2 w 0.494 0.505 0.494 0.6 0.6 AP 46.423.6 46.4 80.1 80.1 L 1000 1000 1000 1000 1000 tfd 0 0 0 0.106 0.106

The performance of the filter was simulated using a 3D finite elementmodeling tool. The curve 1610 is a plot of S21, the input-outputtransfer function, of the simulated filter 1400 as a function offrequency. The filter bandwidth is about 800 MHz, centered at 4.75 GHz.The simulated performance does not include resistive or viscous losses.

A first dielectric layer having a first thickness may be deposited overthe IDT of the shunt resonators and a second dielectric layer having asecond thickness may be deposited over the IDT of the series resonators.The first thickness may be greater than the second thickness. Adifference between an average resonance frequency of the seriesresonators and an average resonance frequency of the shunt resonators isdetermined, in part, by a difference between the first thickness and thesecond thickness.

The first and second filters (whose S21 transmission functions are shownin FIG. 15 and FIG. 16 ) are examples of filters using XBARs. A filtermay use more or fewer than two shut resonators, more or fewer than threeseries resonators, and more or fewer than five total resonators. Afilter may use reactive components, such as capacitors, inductors, anddelay lines in addition to XBARs. Further fine tuning of the individualresonators of these filters may improve filter performance.

CLOSING COMMENTS

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andprocedures disclosed or claimed. Although many of the examples presentedherein involve specific combinations of method acts or system elements,it should be understood that those acts and those elements may becombined in other ways to accomplish the same objectives. With regard toflowcharts, additional and fewer steps may be taken, and the steps asshown may be combined or further refined to achieve the methodsdescribed herein. Acts, elements and features discussed only inconnection with one embodiment are not intended to be excluded from asimilar role in other embodiments.

As used herein, “plurality” means two or more. As used herein, a “set”of items may include one or more of such items. As used herein, whetherin the written description or the claims, the terms “comprising”,“including”, “carrying”, “having”, “containing”, “involving”, and thelike are to be understood to be open-ended, i.e., to mean including butnot limited to. Only the transitional phrases “consisting of and”“consisting essentially of”, respectively, are closed or semi-closedtransitional phrases with respect to claims. Use of ordinal terms suchas “first”, “second”, “third”, etc., in the claims to modify a claimelement does not by itself connote any priority, precedence, or order ofone claim element over another or the temporal order in which acts of amethod are performed, but are used merely as labels to distinguish oneclaim element having a certain name from another element having a samename (but for use of the ordinal term) to distinguish the claimelements. As used herein, “and/or” means that the listed items arealternatives, but the alternatives also include any combination of thelisted items.

It is claimed:
 1. A filter device, comprising: a substrate; apiezoelectric plate having front and back surfaces, the back surface ona surface of a substrate, portions of the piezoelectric plate formingone or more diaphragms, each diaphragm spanning a respective cavity inthe substrate; a conductor pattern on the front surface, the conductorpattern including a plurality of interdigital transducers (IDTs) of arespective plurality of acoustic resonators including a shunt resonatorand a series resonator, interleaved fingers of each of the plurality ofIDTs on a diaphragm of the one or more diaphragms; a first dielectriclayer having a first thickness on the front surface between the fingersof the IDT of the shunt resonator; and a second dielectric layer havinga second thickness on the front surface between the fingers of the IDTof the series resonator, wherein the piezoelectric plate and all of theplurality of IDTs are configured such that radio frequency signalsapplied to the plurality of IDTs excite respective primary shearacoustic modes within the one or more diaphragms, and the firstthickness is greater than the second thickness.
 2. The filter device ofclaim 1, wherein a difference between a resonance frequency of theseries resonator and a resonance frequency of the shunt resonator isdetermined, in part, by a difference between the first thickness and thesecond thickness.
 3. The filter device of claim 1, wherein the firstthickness is less than or equal to 500 nm, and the second thickness isgreater than or equal to zero.
 4. The filter device of claim 1, whereinrespective directions of acoustic energy flow of each of the excitedprimary shear acoustic modes are substantially normal to the surfaces ofthe piezoelectric plate.
 5. The filter device of claim 1, wherein athickness between the front and back surfaces of the piezoelectric plateis greater than or equal to 200 nm and less than or equal to 1000 nm. 6.The filter device of claim 5, wherein each of the plurality of IDTs hasa respective pitch greater than or equal to 2 times the thickness of thepiezoelectric plate and less than or equal to 25 times the thickness ofthe piezoelectric plate.
 7. The filter device of claim 1, wherein thefirst and second dielectric layers comprise at least one of silicondioxide and silicon nitride.
 8. The filter device of claim 1, whereinthe conductor pattern comprises one of aluminum, an aluminum alloy,copper, a copper alloy, beryllium, and gold.
 9. The filter device ofclaim 1, wherein the plurality of resonators includes two or more shuntresonators, and the first dielectric layer is on the front surfacebetween the fingers of all of the two or more shunt resonators.
 10. Thefilter device of claim 1, wherein the plurality of resonators includestwo or more series resonators, and the second dielectric layer is on thefront surface between the fingers of all of the two or more seriesresonators.
 11. A filter device, comprising: a substrate; apiezoelectric plate having front and back surfaces, the back surface ona surface of a substrate, portions of the piezoelectric plate formingone or more diaphragms, each diaphragm spanning a respective cavity inthe substrate; a conductor pattern on the front surface, the conductorpattern including a plurality of interdigital transducers (IDTs) of arespective plurality of acoustic resonators including a shunt resonatorand a series resonator, interleaved fingers of each of the plurality ofIDTs on a diaphragm of the one or more diaphragms; a first dielectriclayer having a first thickness over the IDT of the shunt resonator, anda second dielectric layer having a second thickness over the IDT of theseries resonator, wherein the piezoelectric plate and all of theplurality of IDTs are configured such that radio frequency signalsapplied to the plurality of IDTs excite respective primary shearacoustic modes within the one or more diaphragms, and the firstthickness is greater than the second thickness.
 12. The filter device ofclaim 11, wherein a difference between a resonance frequency of theseries resonator and a resonance frequency of the shunt resonator isdetermined, in part, by a difference between the first thickness and thesecond thickness.
 13. The filter device of claim 11, wherein the firstthickness is less than or equal to 500 nm, and the second thickness isgreater than or equal to zero.
 14. The filter device of claim 11,wherein respective directions of acoustic energy flow of each of theexcited primary shear acoustic modes are substantially normal to thesurfaces of the piezoelectric plate.
 15. The filter device of claim 11,wherein a thickness between the front and back surfaces of thepiezoelectric plate is greater than or equal to 200 nm and less than orequal to 1000 nm.
 16. The filter device of claim 15, wherein each of theplurality of IDTs has a respective pitch greater than or equal to 2times the thickness of the piezoelectric plate and less than or equal to25 times the thickness of the piezoelectric plate.
 17. The filter deviceof claim 11, wherein the first and second dielectric layers comprise atleast one of silicon dioxide and silicon nitride.
 18. The filter deviceof claim 11, wherein the conductor pattern comprises one of aluminum, analuminum alloy, copper, a copper alloy, beryllium, and gold.
 19. Thefilter device of claim 11, wherein the plurality of resonators includestwo or more shunt resonators, and the first dielectric layer is over theIDTs of all of the two or more shunt resonators.
 20. The filter deviceof claim 11, wherein the plurality of resonators includes two or moreseries resonators, and the second dielectric layer is over the IDTs ofall of the two or more series resonators.